mcp: turn enable_mcp_protocol_2026_07_28 on by default (DEX-175) - #39762

Dates in RC snapshots are provisional until the release ships. One
commit is added per snapshot; the final snapshot sets the real dates.
**The `final` snapshot has been applied: the dates below are the real
ones** (Cloud 2026-11-12, Self-Managed 2026-11-13).

The `final` snapshot adds no entries — `v26.50.0-rc.1...v26.50.0` is one
commit, the version bump, and no agent-skills PR merged in the window.
Its substantive work was the dates, a rebase onto `main`, and the three
removals below.

**Rebased onto `main`.** The branch was 107 commits behind and
conflicted: `main`'s `## v26.49.1` section replaced the stale `##
v26.49.0` stub the branch was cut from. Resolved by keeping `main`'s
section verbatim and re-inserting `## v26.50.0` above it; same for the
compatibility YAML (`v26.50`, then `v26.49.1`, then `v26.48.2`).

**Three entries removed as already published on `main`.** The `v26.49.1`
patch train cherry-picked four Console PRs that also merged to `main`
inside this train's `rc.1` window, so both trains classified them
independently and `main` already carries them under `## v26.49.1`:

| PR(s) | Published on `main` in `## v26.49.1` as | Removed from `##
v26.50.0` |
|---|---|---|
| [#40044](#40044),
[#40074](#40074) | the
feature section *"Right size your clusters, using the new Hydration tab
on the console"*, with a screenshot | *"**Diagnose hydration from the
cluster details page**"* |
| [#40511](#40511) |
*"**Freshness and cluster details pages on Self-Managed**"*, covering
both pages | *"**The Freshness page on Self-Managed**"* |
| [#40492](#40492) |
the Bug Fix about the Console's clusters list showing a dash in the Disk
column | *"**Swap usage in the Console's Disk column**"* |

`main`'s wording is the later and fuller treatment in all three cases.
That leaves `## v26.50.0` with **5 Improvements and 7 Bug Fixes**.

Decisions required:

- **MCP protocol default flip**
([#39762](#39762)):
published as an Improvement because `enable_mcp_protocol_2026_07_28` is
`true` at the tag and the flip *is* the change. The PR was held in draft
"until the 2026-07-28 code ships in a release and passes a test on a
deployed environment" — is it confirmed on?
- **Columnar accumulators**
([#40364](#40364)):
omitted as Borderline — the default is `true` but the PR states no
measured user-facing effect. Is there a memory or CPU figure worth
publishing?
- **MySQL snapshot/replication concurrency**
([#40206](#40206),
[#40207](#40207)):
omitted as Borderline — gated on
`storage_persist_sink_description_lookahead`, which is `Duration::ZERO`
at the tag, same as the PostgreSQL counterparts held back in v26.47.0.
If the flag is served on, both halves want announcing together.
- **MCP servers GA**
([#40448](#40448)):
omitted as Borderline — the preview banners are gone, but the GA call
belongs to the release owner. Publish it?
- **Balancer connection metrics**
([#40192](#40192)):
omitted as Borderline — `mz_balancer_connection_status{status="error"}`
changes meaning, so Self-Managed dashboards and alerts on that series
will see a step change. Worth a note?
- **Console query comments**
([#40075](#40075)):
omitted as Borderline — every Console statement's `sql_hash` changes
once when this deploys. Worth calling out?

Every flag above was re-read in source at the `v26.50.0` ship tag; not
one default moved between `rc.1` and ship, so all six holds still read
correctly.

Snapshot drafts (with PR links) in
[mz-skills](https://github.com/MaterializeInc/mz-skills):

- rc.1:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/rc.1/release_notes.md),
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/rc.1/omitted.md)
- final:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/final/release_notes.md),
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/final/omitted.md)

**Borderline PRs omitted:** rc.1: 7 —
[omitted.md#borderline](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/rc.1/omitted.md#borderline),
final: 0 —
[omitted.md#borderline](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.50.0/final/omitted.md#borderline)
(no PRs in scope; `rc.1`'s six holds re-examined at the ship tag and
unchanged)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <[email protected]>
Co-authored-by: Pranshu Maheshwari <[email protected]>
Co-authored-by: Valerie Chiang <[email protected]>